IP spoofing is an attack launched against a machine playing on the weaknesses of authentication systems. Far too many systems on the Internet rely on IP addresses for their authentication. This is, of course, bad news in that IP addresses provide identification, not authentication. In addition, far too many routers and gateways will route packets with "bad" source IP addresses; (i.e. they will accept and deliver a packet in which the source IP address and the destination IP address should be on the same side of the router). This is a problem for any type of TCP/IP system, including Windows NT.